pumping stations design -...

Pumping Stations DesignFor Infrastructure Master Program

Engineering Faculty-IUG

Lecture 4: Pumping Hydraulics

Dr. Fahid Rabah

Water and environment Engineering

[email protected]


The main items that will be studied under the pumping hydraulics title are:

Pumping pressure and head terminology Cavitation Pump characteristic curves Multiple pump operation

2

Multiple pump operation Variable speed pumps Affinity laws Pump selection


3.6 Pumps selection ( see Chapter 25)

Use of specific speed in pump type selection:

Specific speed is defined as:“The speed of an ideal pump geometrically similar to the actual pump, which whenrunning at this speed will raise a unit of volume, in a unit of time through a unit of

Specific speed is a term used to describe the geometry (shape) of a pump impeller.

3

4

3

t

2

1

s

H

QnN

running at this speed will raise a unit of volume, in a unit of time through a unit ofhead."

Specific speed is calculated from the following formula:

Where:Ns = specific speed (or type number)n = pump rotational speed, rpmHt = total pump head, mQ = pump discharge, m3/s


3.6 Pumps selection

4


3.6 Pumps selection

Use of specific speed in pump type selection:

Example:A flow of 0.01 m3/s a against a head of 20m. The pump is to be driven withAn electric motor at a speed of 1750 rpm. What type of centrifugal pump shouldBe selected and what is corresponding efficiency?

5

5.18

)0

)01.0750

4

3

2

1

4

3

t

2

1

s

(2

(1

H

QnN

Solution:Calculate the specific speed of the pump.

Using figure 10-8 the pump is radial flow with an expected efficiency of 62%


3.6 Pumps selection

1. Type of the liquid to be pumped “ water, wastewater, sludge”.2. Type of installation preferred“ submersible or dry”.3. Pumps available in the market ( KSB, FLYGT, ABS,...).

In the selection process the following important information is needed:

selection of the specific pump needed :

6

selection of the specific pump needed :

1. Draw the system curve envelope and decide the operating point.2. Decide the number of pumps needed ( see table 3.1).3. Use the catalog of the manufacturers to select a suitable pump curve.4. Draw the pump characteristics curve over the system curve.5. Select a pump so that the Operating range of is within 60% to 115% of Q at BEP.6. Compare pumps of different manufacturers for more economical choice.


3.6 Pumps selection

FlygtKSB

7

ABS

KSB


3.6 Pumps selection

Standby pumpsNo. of pumpsneeded

Design flow

m3/h

11Up to 160

Table 3.1 Recommended number of pumps in operation and their stand by.

8

12 or 3160 -450

23 to 5More than 450

Lecture 4: Water hammer phenomena (Hydraulic Transient)

4.1 General Introduction (See chapters 6 &7) What is water hammer?

Water hammer is a phenomena that occurs in pressurized pipe systemswhen the flowing liquid is suddenly (instantaneously) obstructed by(for example) closing a valve or by pump stoppage (failure). A laudnoise similar to hammer knocking noise occurs due to the collision ofthe liquid mass with the obstruction body (valve or pump ) and the

9

the liquid mass with the obstruction body (valve or pump ) and theinternal walls of the pipe.

What is water hammer scientifically?

Water hammer is a descriptive name of the phenomena but not scientific.The scientific name is hydraulic transient. Hydraulics Transient meanstemporary hydraulics since the phenomena last for a very short time.During this short time the flow in the pipe is disturbed and consecutivewaves will be moving back and forth creating a very high pressure risein the pipe.


4.1 General Introduction (See chapters 6 &7)

What is the problem with water hammer?

Hydraulic transient (water hammer) may cause disasters in pressurized liquidsystems such as:1. Rupture of pipes and pump casings.2. Vibration and high noise.3. Excessive pipe displacements.

10

3. Excessive pipe displacements.4. Vapor cavity formation.5. Environmental pollution.6. Life and economical losses.

When water hammer occurs?The most common cases where water hammer occurs are:1. Pump failure.2. Pump start up.3. Sudden closure a valve.4. Failure of flow or pressure regulators.

4.2 Water hammer pressure surge


To illustrate the pressure waves resulting from the hydraulic transient we will discussa tank, pipe, and valve system as an example. The figure below shows the normaloperation of the system before the valve closure.

11

H0

What happens when the valve is closed instantaneously ?

When a valve is closed instantaneously the liquid next to the valve comes to a halt. This liquid is then compressed by the liquid upstream which is still flowing. This compression causes a local increase in the pressure on the fluid. The walls of the pipe around the fluid are stretched by the resulting excess pressure. A chain reaction then takes place along the length of the pipe, with each stationary



12

A chain reaction then takes place along the length of the pipe, with each stationary element of fluid being compressed by the flowing fluid upstream. This chain reaction results in a pressure wave which travels up the pipe with a

velocity “a “, which is called the celerity. (it may reach the speed of sound inwater (1484 m/s).

The pressure wave creates a transient hydraulic grade line (HGL) which is parallelto the steady flow HGL, but at a height above it.

The resulting increase in pressure is due to the water hammer surge.When the pressure wave reaches the reservoir the pressure cannot exceed

the water depth in the reservoir.

What happens when the valve is closed instantaneously ?

A wave of pressure unloading travels back along the pipe in the opposite direction. As the unloading wave travels down the pipe the flow is reversed, toward the

reservoir.When this unloading wave hits the closed valve the flow is stopped and a drop

in pressure occurs.



13

in pressure occurs. A negative pressure wave now travels up the pipe toward the reservoir. The process described above now repeats itself for a negative wave. A cycle of pressure waves (positive - unloading - negative - unloading)

now travels up and down the length of the pipe. Friction has the effect of slowly damping out the effect of the pressure waves.When the negative pressure wave travels along the pipe very low pressures

may cause cavitation

What are the factors affecting the wave speed and How it can be calculated ?

The speed of pressure wave “a” depends on :

the pipe wall material.

the properties of the fluid.

the anchorage method of the pipe.



14

For elastic pipes (steel, PVC,..) the wave speed is given by the followingformula:

e

D

E

KC

K

a

1

What are the factors affecting the wave speed and How can it can be

calculated wave ? Continued …….

Where:

a = elastic wave speed in water contained in a pipe, m/s

K =Bulk modulus of elasticity of the liquid, N/m2 ,(K= 2.15X109 N/m2 for water)



15

E = Modulus of elasticity of pipe material, N/m2

D = Inside diameter of the pipe, m

e = Wall thickness of the pipe, m

ρ = Density of the fluid , kg/m3

C = Correction factor depending on the type of pipe restrain

= (1.25 -µ) for pipes anchored from one side only.

= (1.0-µ2) for pipes anchored against axial movements.

= 1.0 for pipes with expansion joints throughout.

µ = Poisson’s ratio of the pipe material



What do we mean by instantaneous valve closure?

The time required for the pressure wave to travel from the valve to the reservoirand back to the valve is:

Where:

L = length of the pipe (m)

a

Lt

2

16

L = length of the pipe (m)

a = speed of pressure wave, celerity (m/sec)

If the valve time of closure is tc , then

If the closure is considered gradual

If the closure is considered instantaneous

a

Ltc

2

a

Ltc

2



What is the value of the pressure surge?

There are two cases for pressure surge:1. Gradual closure of the valve:

In this case and the pressure increase ( Δ H) is:a

Ltc

2

Where:

17

2. Instantaneous closure of the valve:

In this case and the pressure increase ( Δ H) is:

tg

VLH 0

g

VaH 0

a

Ltc

2

Where:V0 = initial velocity of the fluid , m/sg = gravitational acceleration, m2/st = closure time, sL = pipe length, ma = Wave speed, m/s

Note: that (Δ H) due to gradual closure is very

small compared with the instantaneous (Δ H).

Lecture 4: Water hammer phenomena


18



19



20

a

ΔH



21

ΔH

H0

V0

V=0.0



a

22

- ΔH

H0

V0

V=0.0

a

ΔH

The time history of the pressure wave for a specific point on the pipe is a graphthat simply shows the relation between the pressure increase ( ΔH, or ΔP ) and Time during the propagation of the water hammer pressure waves. The followingFigures illustrate the time history in the tank pipe system discussed previously.


4.3 Time history of pressure wave due to water hammer):

23



The figure below is the time history for a point “A” just to the left of the valve.The friction losses in the pipe are neglected.

24

Time history for pressure at point “A” (after valve closure)

aa



The time history for point ”M“ (at midpoint of the pipe). The friction losses in thepipe are neglected.

2a

25Time history for pressure at point “M” (after valve closure)

a



The time history for point B (at a distance x from the reservoir ) The friction lossesin the pipe are neglected.

a

26Time history for pressure at point “B” (after valve closure)



In real practiceIn real practice friction effects are considered and hence adamping effect occurs and the pressure wave dies out, i.e.;energy is dissipated.

Damping effect of friction

27

Time history for pressure at point “A” (after valve closure) when friction lossesare included.

t*(2L/a)


4.4 water hammer in pumping mains

Water hammer due to power failure:

As illustrated previously,one of the seriousevents that producewater hammer indelivery pipes of

28

delivery pipes ofpumping stationsis power failure.



Column separation due to power failure:

- What do we mean by column separation?

-After power failure a wave of negative pressure “down surge” occurs.- If the hydraulic profile intersects the pressure pipe and falls below it for apressure less or equal to the vapor pressure, water vapor starts to accumulateAnd large air pockets are formed.

29

And large air pockets are formed.-The air pocket separates the water column in the pipe into to parts, upstreamcolumn and down stream column. This phenomena is called column separation.-Column separation is dangerous when the two water columns join again withhuge collision when the reflected wave comes back. This collision produce anupsurge pressure that is much more than the first water hammer impulsecalculated from the equation discussed previously .

g

VaH 0


4.4 water hammer in pumping mains Column separation due to power failure:

- How to predict column separation?

30

To predict the occurrence of negative pressure and column separation due towater hammer draw a mirror image of the normal hydraulic profile . If the mirror HGLfalls below the pipe for a distance “d “ = hatmospheric – hvapor, thin column separationwill occur.



a

Graphical solution to calculate the water hammer surge after power failure:

A. water hammer without column separation:

The figure in the next slide illustrates the surge development in a pumpingmain after power failure. The following formulas are needed to understandthe method:

31

pgA

atan

10A : the subscripts 0-1 means the situation at point Abetween time 0 and time 1, the same notation is used for point B.Time 1 in this example is = L/a.

: Λ (Lambda) is the angle of inclination of the surge line PR, Ap

is the cross section area of the pressure line.

tan2 0

0

H

QC p

: Cp is the pipe constant, (Q0,H0) is the operating pointbefore pump failure.

If Cp is <0.50 then the maximum pressure at the pump is positive as this case.If Cp is > 0.50 then the maximum pressure at the pump is negative and columnseparation will occur as the next case explained in B.


4.4 water hammer in pumping mains Graphical solution to calculate the water hammer surge after power failure:

B. water hammer with column separation:

As indicated in Case A if Cp is > 0.50 then column separation occurs.The following example illustrates this case.

33


4.4 Prevention of water hammer in pumping mains

Methods of preventing or minimizing the water hammer surge:

A. Control Tanks:- Open end surge tank or standpipe- one way surge tank- two way surge tank- Air champers (hydropneumatic tanks)

35

- Air champers (hydropneumatic tanks)

B. Control valves:- Vacuum relief valve- Air release valves- Air release and vacuum relief valves- surge relief valves



36



37

Horizontal Air chamber for water hammer prevention



38

Horizontal Air chamber forwater hammer prevention

Vertical Air chamber forwater hammer prevention



39surge relief valve



40

Air release valve



41

Vacuum relief valve Air /Vacuum relief valve



Using Air champers to prevent water hammer surge:

42

Fig 80



43

Charts to determinethe size of theair champers

0

0

2gH

aV

orificechamperair

in thepercentlossesK

orificechamperair

in thepercentlossesK

44

Fig 83

champerair

theinvolume

aircompressed

initialThe0C

min

00/

H

HCC

pumpthetoadjacentsurgedownHH 0min

: air chamber volume



Using computer software for water hammer analysis:

There are many computer software for water hammer analysis such as:

- Hammer (Bently company)- HiTran- AFT

45

- AFT- Pipe Expert

These software are not free , they are very expensive. However, you canDownload DEMO versions and get good benefit out of it. You can also searchFor free software that I do not know.

Lecture 5: Design of wastewater pumping stations

5.1 General introduction

Main components of WWPS:

- Bar screen- Grit removal- Wet well or wet well + dry well- Electricity distribution and control room (MDB + PLC)- Transformer room

46

- Transformer room- Stand by generator and its fuel tank- Guard room and its services (kitchen + showers and toilet)- Pressure pipes and control valves- Fence and landscaping

pumping stations design -...

Documents